The present invention relates to an improvement in the adhesion and corrosion resistance of painted film applied over chemical conversion coatings on zinc or alloys thereof.
It has been well known that metallic articles having a surface of zinc or zinc alloy exhibit poor paint adhesion. In order to improve the adhesion, the surface has been treated to form a phosphate coating or a complex oxide coating. It has also been well known that a chromate coating applied on the surface of zinc or zinc alloy shows high corrosion resistance but poor adhesion of painted film and poor resistance against scratching as compared with phosphate coatings. In order to improve further the corrosion resistance of phosphate conversion coatings or complex oxide coatings, such coatings have been subjected to a post-treatment process with chromic acid in which the conversion coatings are treated with chromic acid, a dichromate or salts thereof.
The post-treatment process with chromic acid is inexpensive and affords excellent corrosion resistance. However, the toxic and deleterious environmental properties of chromium compounds have recently become serious problems. In order to alleviate the dangers of chromates, it would be desirable to eliminate the use of chromate compositions without sacrificing quality.
Post-treating compositions containing no chromate include those containing predominantly phytic acid as claimed in Japanese Patent Publication No. 43406/1973, those containing predominantly alpha-aminophosphoric acid or alpha-aminosulfonic acid as claimed in Japanese Patent Publication No. 78531/1975, aqueous solutions containing free fluoride ion or a complex ion thereof as claimed in U.S. Pat. No. 3,895,970 and tannic acid as claimed in Japanese Patent Publication No. 2902/1976. The corrosion prevention of metals with tannin has been studied for some years and reported by a number of investigators, especially by E. Knowles, T. White and the like. Such reports are summerized by Journal of the Oil Colour Chemistry Association, Vol. 41, pp. 10 (1956). While some effect can be achieved by such processes, the performance on zinc and alloys thereof is poor as compared with the post-treatment process with chromic acid. The application of such post-treatments over phosphate coatings on zinc may cause some deterioration of the corrosion resistance after painting and, in particular, may cause blisters in painted film as compared to the case where no post-treatment is employed.